This invention relates to a semiconductor light detector device, and more particularly to such a device adapted to a light detector for spectrophotometry, etc.
In conventional spectrophotometry, the light energy from a light source is dispersed or separated into its various components at different wavelengths through a suitable optical system. The dispersed light components having particular wavelengths are successively passed through a sample. The amount of each light transmitted through the sample is detected by one semiconductor light detector.
A recently proposed spectrophotometry technique uses as a light detector device an array of photodiodes which simultaneously receive the dispersed light components. The output signals from the respective photodiodes are successively delivered.
FIG. 1A of the accompanying drawings shows the circuit arrangement of a charge storage type light sensor as an example of such a photodiode array detector. FIG. 1B shows the structure of that part of FIG. 1A enclosed by a dotted block. For the illustration of FIGS. 1A and 1B, one can refer to U.S. Ser. No. 6,913 filed on Jan. 26, 1979, now U.S. Pat. No. 4,242,695, and assigned to the same assignee as the present application, in which a similar illustration is shown.
Referring to FIG. 1A, symbols PD.sub.l -PD.sub.n designate n photodiodes which are arranged in correspondence with the dispersed light components or wavelengths, symbols T.sub.Sl -T.sub.Sn switches which include MOS transistors, symbol SR a scanning circuit which includes shift registers SR.sub.l -SR.sub.n, symbol R.sub.L a load resistor, and symbol V.sub.O a power supply. The switches T.sub.Sl -T.sub.Sn connected in series with the respective photodiodes PD.sub.l -PD.sub.n respond to sampling gate pulse signals from the shift registers SR.sub.l -SR.sub.n to perform the on-off control of a circuit which includes each photodiode PD, the load resistor R.sub.L and the power supply V.sub.O.
Referring to FIG. 1B showing in cross section the conventional structure of a photodiode PD and a switch T.sub.S which constitute one bit of the circuit shown in FIG. 1A, a p-type light receiving region 2 of the photodiode PD and a p-type source region 3 of the switch T.sub.S are contiguously formed in one surface of an n-type semiconductor layer 11 as an n-on-n.sup.+ type semiconductor substrate 1. Further, there is formed a p-type drain region 4 of the switch T.sub.S with a channel region 5 interposed between the source and drain regions 3 and 4. Reference numeral 8 designates an electrode of the photodiode PD formed on the surface of the semiconductor substrate 1 at the n.sup.+ -type layer, numeral 6 a gate electrode of the switch T.sub.S formed on the channel region 5 through an SiO.sub.2 film 9, numeral 7 a drain electrode of the switch T.sub.S formed on the drain region 4, and numeral 91 a surface protection or field oxide film. Usually, n couples of such photodiode PD and switch T.sub.S are fabricated in monolithic or hybrid IC configuration.
In operation, dispersed incident light components having different wavelengths impinge upon the photodiodes PD.sub.l -PD.sub.n so that charges proportional to the amount of the incident light are stored at a photosensitive pn junction of each photodiode PD. Then, the switches T.sub.Sl -T.sub.Sn are successively turned on in response to sampling gate pulses from the shift registers SR.sub.l -SR.sub.n. The turn-on of one switch T.sub.S forms a closed series circuit which includes associated photodiode PD, the load resistor R.sub.L and the power supply V.sub.O. By the formation of this series circuit, a charging current corresponding to the quantity of charges stored at the pn junction of the photodiode PD is supplied from the power supply V.sub.O through the resistor R.sub.L across which an output voltage is developed.
FIG. 1C shows the equivalent circuit of the charge storage type light sensor shown in FIG. 1A. In FIG. 1C, symbol C.sub.P designates the junction capacitance of photodiode PD, symbol C.sub.D a stray capacitance which includes the drain junction capacitance of switch T.sub.S, the wiring or interconnection capacitance, etc., and symbol R.sub.ON the resistance of switch T.sub.S in its turned-on state. The stray capacitance C.sub.D is an important factor giving a great influence on the signal output characteristic of the light sensor. The reasons are as follows.
(1) Since the charging current to the junction capacitance C.sub.P of the photodiode PD contributing to photo-electric conversion may be supplied by charges stored in the stray capacitance C.sub.D as well as the power supply V.sub.O, a large value of the stray capacitance C.sub.D results in the decrease of the charging current through the load register R.sub.L, thereby lowering the level of a signal output voltage developed across the resistor R.sub.L.
(2) The gate pulse applied to the switch T.sub.S from the shift register circuit SR may cause leakage through a capacitance between the gate and the drain, thereby generating large spike noises. A large value of the large stray capacitance C.sub.D results in large leading and tailing time constants of the noise pulses so that significant signals are buried in the large noise pulses, thereby rendering the signal detection impossible.
To attain the signal detection with high S/N ratio, it is necessary to separate or discriminate signals from noises. One conventional approach for that purpose was to juxtapose an additional counterpart with the above-described actual light sensor, the additional counterpart having the same configured photodiode, switch and shift register arrangement as the actual light sensor excepting that the photodiodes in the additional counterpart have their screened light receiving portions insensitive to light. Signals derived from adjacent juxtaposed photodiodes in the actual light sensor and the additional counterpart are passed through a differential amplifier to eliminate spike noises. This proposed construction is very complicated.